1. Field of the Invention
The present invention relates to a downstream plasma processing apparatus used for an etching or ashing process during fabrication of an integrated circuit semiconductor (IC) device or similar such devices. More particularly, the present invention relates to a downstream plasma microwave etching apparatus having an improved coupling structure between a waveguide and a plasma generating region for providing substantially uniform etching or ashing of an IC substrate or a similar workpiece to be processed, along with stable impedance matching of the microwave circuit, and which allows uniform high rate downstream plasma processing of a semiconductor material by reducing the reflection of microwaves.
2. Description of the Related Art
A known process for forming fine patterns in an IC device employs both an etching process and an "ashing" process, i.e., a process of etching off the protective layers of silicon, silicon dioxide, or silicon nitride formed on a semiconductor substrate, and a process of stripping, or "ashing," a photoresist mask, consisting of a layer of an organic material, off the substrate.
Recently, "wet" etching, utilizing chemical solvents, has been replaced by "dry" etching which affords various advantages, such as the capability of achieving fine resolution and less undercutting of the etched pattern, elimination of wafer handling for rinsing and drying, inherent cleanliness and the like. In particular, by employing a plasma process, sequential etching and ashing operations with the same apparatus become possible, allowing implementation of a fully automated fabricating process for IC devices.
A plasma is a highly ionized gas containing nearly equal numbers of positively and negatively charged particles plus free radicals. The free radicals are electrically neutral atoms or molecules that can actively form chemical bonds with other atoms. The free radicals generated in a plasma, acting as a reactive species, chemically combine with a material to be etched, to form volatile compounds which are removed from the system by evacuation. Such plasma etching is referred to as reactive ion etching (RIE). A plasma etching apparatus includes essentially a plasma generating region, a reacting region and an evacuating device.
In a plasma etching apparatus, a plasma is generated, typically in a gas at a pressure of approximately 0.05 to 2 Torr, by applying radio-frequency, or microwave, energy to the gas. The generating efficiency of the plasma, or the level of chemical radicals contained in the plasma, is an important factor in plasma processing. Microwave plasma etching and ashing provide the advantages of high plasma density and electrodeless discharge, and are widely used for the fabrication of IC devices.
There are two basic types of plasma processing. One is the so-called "oven type" plasma processing, where a workpiece is placed in a plasma generating region and exposed to the plasma. The other type is the so-called "downstream" plasma processing where the workpiece is placed outside the plasma generating region. Generally, an IC device exposed in a plasma environment, such as in an oven, is damaged by ion bombardment, radiation of ultraviolet rays, soft X-rays, and the like, and the IC substrate should be shielded from the plasma. Therefore, downstream plasma processing is preferred for an IC substrate because it provides the needed shielding. Hereinafter, the term "plasma processing" refers only to downstream plasma processing, unless otherwise indicated.
Radicals generated in a plasma are introduced into a reaction region to react with a material to be worked, which is placed therein. Unfortunately, the radicals are believed to collide with other gas molecules or wall surfaces, resulting in recombination and loss of chemical activity. It is, therefore, very important to reduce the recombination of the radicals. Taking the above need into consideration, an improved microwave plasma etching apparatus has been developed, disclosed in U.S. Pat. No. 4,512,868, issued on Apr. 23, 1985, to S. Fujimura et al. and is incorporated by reference herein. FIG. 1 is a schematic cross-sectional view of the microwave plasma etching apparatus disclosed by the above patent. Through a waveguide 1, microwave power 2 supplied from a microwave energy source 12 is transmitted in the direction indicated by arrow A. An evacuatable processing vessel 4, including a plasma generating chamber 9 and a reaction chamber 10, is partitioned from the waveguide 1 by a dielectric window 3, such as a ceramic or a silica glass window. The plasma generating chamber 9 is effectively part of the waveguide 1. The reaction chamber 10 is spatially contiguous with the plasma generating chamber 9 and delineated therefrom by a metallic mesh 6, which defines and forms a portion of the wall of the waveguide 1. The metallic mesh 6 acts as a microwave energy shield, to prevent the plasma generated in the chamber 9 from passing into the reaction chamber 10, and thereby protects workpiece 11 which is supported on a platform or stage 5 in the reaction chamber 10. Even while shielding the microwave energy, the metallic mesh 6 permits the radicals generated in the plasma to pass into the reaction chamber 10.
The successive processing steps are as follows. The processing vessel 4, including the plasma generating chamber 9 and the reaction chamber 10, is evacuated by a pumping device (not shown) through exhaust tubes 7. A reactive gas is introduced into the plasma generating chamber 9, through a gas supply tube 8, and is ionized by the microwaves 2 to form a plasma. Radicals generated in the plasma pass through the metallic mesh 6 and reach the object 11, such as a semiconductor substrate mounted on the stage 5, and react with the object 11, producing volatile compounds which are exhausted by the pumping device. Thus, the plasma processing operation is carried out in the reaction chamber 10 downstream of the plasma. Accordingly, the above-described plasma processing is referred to as downstream plasma processing. The distance between the plasma generating chamber 9 and the object 11 may be as small as approximately 8 mm, for example. This short distance permits radicals to survive transmission from the plasma generating chamber 9 to the reaction chamber 10. As a result, the workpiece 11 is processed effectively and a relatively high plasma processing rate is attained.
As can be seen from FIG. 1, the dielectric window 3 is disposed in a direction perpendicular to the direction A in which the microwaves 2 are transmitted or propagated. In other words, the window 3 is disposed parallel to the electric field of the microwaves 2. In this configuration the microwaves 2 are partially reflected by two boundary layers or sharp transitions of the dielectric constant, in the transmitting direction A of the microwaves 2. The first transition occurs at the interface between the air space within the waveguide 1 and the dielectric window 3, and the second occurs at the interface between the dielectric window 3 and the plasma generating chamber 9 which is a vacuum space or a plasma-filled space, depending on the processing step. In addition, the electrical impedance of the plasma generating chamber 9 varies substantially depending on the absence or presence of the plasma therein. This imposes a serious obstacle to proper impedance matching of the relevant microwave circuit for all conditions in the plasma generating chamber 9. If the impedance is matched while the plasma generating chamber 9 is maintained in a vacuum, i.e., is evacuated, there will be a mismatched impedance during and after the generation of the plasma in the chamber 9, because reflection of the microwaves 2 is substantially increased upon the generation of the plasma. In one test of the apparatus illustrated in FIG. 1, oxygen was employed as the reactive gas at a pressure of 1 Torr. In this test, 70% of the microwaves 2 were reflected without matching and 30% with matching. Mismatching of the impedance in the microwave circuit may result in a poor production of plasma, and sudden and large reflection of microwave energy which can damage the apparatus.
Another problem is the decay of the microwaves in the plasma generating chamber 9. The microwave electrical energy 2 is consumed in ionizing the reactive plasma gas molecules when introduced into the plasma generating chamber 9, and decays rapidly. Consequently, the intensity of the microwave electric field 2 drops rapidly as the microwaves 2 progress in the direction A in the plasma generating chamber 9; as a result, the distribution of plasma density P drops rapidly, as shown by line Pd of FIG. 1.
When long life radicals, such as fluoride radicals generated from carbon tetrafluoride (CF.sub.4) gas, are utilized, the radicals reach the workpiece 11 substantially uniformly, because the radicals are capable of surviving until reaching the workpiece 11 despite repeated collisions with other particles and vessel walls and resultant changes in direction. However, with respect to short life radicals, such as oxygen radicals which are utilized for ashing a photoresist layer, the radicals capable of reaching the object are limited substantially to those which, when generated, were directed towards the workpiece 11. As a result, the rate of plasma processing of the workpiece 11 is not uniform with a distribution similar to that of the generation of the radicals in the plasma. Thus, uniform plasma processing of a workpiece 11, particularly one having a wide area such as a photoresist layer coated on a large size silicon wafer, is very difficult.
Several methods have been proposed to solve the above-described problem. For example, in an ashing process for removing a photoresist layer disposed on a semiconductor substrate 11, a difference is produced between the respective pressures of the reactive gas, namely oxygen gas, in the plasma generating chamber 9 and the gas in the reaction chamber 10, to provide a uniform generation of radicals in the plasma generating chamber 9. However, the elevation of oxygen gas pressure in the plasma generating chamber 9 induces further, frequent collisions between the oxygen radicals and oxygen gas molecules, reducing the active life of the radicals producing a poor ashing rate, in practice. It is also possible to add carbon tetrafluoride gas (CF.sub.4) to the oxygen (O.sub.2) gas to extend the active life of the oxygen radicals. In this method fluoride radicals are also generated, which damage the circuits and layers on the substrate.
In addition to the problems discussed above, a similar problem is caused by the rapid decay of the microwave energy during its transmission through the plasma. To overcome this problem, an improved configuration of the waveguide has been disclosed in Japanese Laid Open Pat. application No. 61-131454 of S. Fujimura et al., provisionally published on June 11, 1986 which corresponds to U.S. Pat. application Ser. No. 802,332, filed Nov. 28, 1985. The '454 published patent application discloses a processing vessel with a waveguide having a dielectric material microwave transmission window disposed perpendicular to the microwave electric field in the waveguide. In this configuration, the mode of the microwave transmission from the waveguide to the reaction chamber (i.e., a reactor) is not adversely affected, and the microwave energy is effectively absorbed in the plasma. However, the apparatus disclosed in the '454 published patent application is an "oven type" plasma processing apparatus, and is essentially different from a downstream plasma processing apparatus in which a workpiece is placed outside the plasma and processed by transported radicals. Therefore, to overcome the above-described problems of nonuniform and instable plasma processing, a further improved downstream plasma processing apparatus for semiconductor fabrication is desired.